1. Field of the Invention
The present invention relates to telecommunications networks and more particularly to signalling protocols therefore.
2. Related Art
Traditionally public switched telephone networks (PSTN) comprise major switching nodes which are fully interconnected to each other and local switching units which depend from one or more of the major switching nodes. The local switching units are connected by a pair of electrical conductors to each point at which telephony service is required.
When a call is set up through the network a local switching unit effects a connection to the appropriate pair of conductors and applies an ac electrical signal which causes an alert at the consumer's premises.
Originally, the alert was an electromechanical bell or buzzer while later electronic tone callers were used. Because of the nature of the alert other devices such as facsimile machines, answering machine and computer modems were all adapted to respond to the signal provided to operate a bell.
Similarly, as PSTNs evolved, the required response to ringing current was a line loop which caused an electromechanical relay to operate in the exchange, disconnecting the ringing current. Now, although some forward signalling capability developed once the line was looped (for example dual tone multi-frequency signalling of digits rather than loop-disconnect signalling) PSTN signalling in the so called local loop remained at a fairly primitive level.
This being the case, most of the electrical conductors from the local switch unit are unused for much of the time since, clearly, any ringing current applied to a line would result in telephone bells ringing disturbing the household.
The assignee of the present invention has now proposed extending the use of PSTN to include telemetry and other applications of the local loop and has accordingly developed certain signalling protocols for use on consumer connections.
Previous proposals for dealing with telemetry in the local loop have included additional equipment in the local exchange and trunk overlay networks. Such equipments are expensive to provide. For example, in EP474407 there is described a telemetry system including special no-ring trunks. Other disclosures of a specific no-ring trunk arrangement may be found, for example in Shirai et al NEC Research and Development number 65, pages 82-92 an article entitled "Remote Automatic Meter Reading and Telecontrol System Using Telephone Lines".
A similar system was discussed at the International Conference on Communication November, 1984 as published in the Proceedings at pages 2192-2193 in an article entitled "New Telecommunications Service for Telemetering" by M Kurachi et al.
Providing additional overlays and re-equipping all exchanges to which telephone customers may be connected is an extremely difficult and expensive operation and if the existing telecommunications network in its entirety could be used for providing no-ring call service without substantial modification many differing services could be readily provided at minimum additional cost.